M20 Conversion Basics

Could there be an easier engine swap than an E30 325i M20 into a E21 323i? Actually, yes: an S52 into an E36 is pretty plug and play. But this almost as easy. There are a few really good guides online, including the ones covering 320i swaps. They didn’t always apply to my project so here’s what I learned along the way.

First, here are the best resources I found:

Todor’s: http://www.todor.info/repair/e21swap/

bf.com thread: http://www.bimmerforums.com/forum/showthread.php?1045649-How-to-swap-M20s-E21-323i-to-Motronic-E30-325i

The swap is easy enough that you don’t need a lot of how-to. This is what you will need:

Complete M20 engine with wiring and ECU (get it as complete as you can)

E30 air flow meter

E30 cone filter intake (the E30 air box might fit but I didn’t try)

E30 throttle cable

New rubber fuel lines from the frame rail to the E30 fuel rail.

E21 fuse box plug and pins

E21 fuse box plug

E30 C101 connector. The top should be on the engine wiring but get the body-side for a factory look.

If you have a 1988+ M20 you will need the 1987 cooling system parts (water pump and hoses). Some of the hoses can come from the E21 system.

An extra E30 front exhaust manifold. The E30 exhaust layout has the manifold outlets next to each other. The E21 has them spread apart. This might vary from car to car, however. I could have used the E21 manifolds but they looked too rusted on the studs. On my car I could not use the E30 rear manifold and had to source a front one. Everything bolted up from that point.

E30 oxygen sensor and bung. And realize that the bung cannot be welded to a rusty exhaust pipe.

You will probably want to relocate the battery to the trunk. That required a battery tray, battery cable, junction box for the engine bay, and wiring from the junction to the starter and alternator. And you will need to cut several holes in the interior sheet metal.

You also need to cut a hole in the firewall for the engine ECU so it can mount under the dash, a la E30.

The E21 flywheel and clutch will bolt on to the E30 M20. Some of the parts are actually the same anyway.

Make sure the engine is complete and running. If you buy a non-running engine upfront you will be chasing those issues later instead of driving it. I chased a no-start issue for weeks because the engine I bought needed basic maintenance. Here is what tripped me up once the engine was in the car:

  • fuel pump relay
  • short in the fuel rail
  • alternator voltage regulator
  • alternator ground
  • TPS adjustment
  • throttle stop screw

They’re not expensive so make sure this stuff is taken care of.

You will need the motor mount arms from the E21 (which you won’t get until the engine is out of the car).

As for the actual swap itself, I did it in my driveway with a Harbor Freight engine hoist. You have to pull the hood off and the latch mechanism and rods need two people to safely remove.

I pulled the engine with the transmission attached but it was SCARY. I won’t be doing that again. It came out pretty well but the two pieces together made it a struggle to reinstall.

Here is the wiring that needs to be done:

oil pressure light in instrument cluster
fuel pump power
alternator charge light in cluster
ignition coil power
ECU power
power to starter
coolant housing temp to cluster
RPM signal to tachometer. This was the only connection that did not work. I ended up running a dedicated wire from the coil to the C101 plug (bypassing the E30 ECU).

Here is my wiring inventory:

I was very pleased with the results and how straight-forward this was. It was a great learning experience and I’m enjoying the fruits of my labor.

in it’s new home

Here are the links to my engine build and pics from the swap: http://douging.smugmug.com/Cars/BMW-M20-Engine-Restoration


6 thoughts on “M20 Conversion Basics

Add yours

  1. Hi Doug,
    Recently I bought a E21 with M20B25 engine converted by the previous owner.
    However I noticed that the speedometer and milage counter is not functioning. Upon checking I noticed that there’s no cable coming from the back axle to the dashboard.
    Do you face same issue when you did your conversion?
    Hope you could share and help me.
    I can be reached at dankhar@yahoo.com



    1. Hello, The engine likely came from an E30 along with the ECU and transmission. The E30 used an electronic signal from a sensor in the rear axle to show speed on the speedometer.

      But the E21 uses a mechanical connection between the back of the transmission and the speedometer. I don’t believe the E30 transmission can be modified to use the shaft that drives the cable.

      You can:
      A) find a true E21 323i or E12 520i Getrag transmission and it will have the cable hookup. Note: don’t buy a E21 320i transmission as the starter housing is in the wrong place.
      B) install an E30 speed sensor on the rear axle and convert it to mechanical. Something like this adapter: https://www.amazon.com/Koso-BF580001-Speed-Signal-Converter/dp/B01ISPDLSG
      C) install an E30 speed sensor on the rear axle and an E30 cluster in the dash

      Hope the engine conversion is running well otherwise!


      1. Hi Doug,
        Looks like the option C is the best way to explore as my cluster is already an E30.
        However, the speed sensor that you have suggested still needs to be connected to another longer cable to reach to the cluster?


        1. I have the cable from the cluster going to the back of the transmission (it might be a 2-piece cable). So I assume you would extend the wiring from the new sensor to meet the cluster cable. You may be able to locate the sensor on the driveshaft if it can read accurately.


  2. Hi Doug
    I appreciate if u can guide me on option B from above .
    I replaced my e21 320/6 transmission years ago with one from an e30 , as u know the kilometer not working due to no mechanical port on e30 transmission.
    Can you go in mote detail how that sensor converter works to allow my kilometer work again?
    Where exactly it will be installed?
    How you adapt to e21 original cable ?


    1. I never needed to use a converter. I was lucky to grab some correct E21 323i transmissions so no tricks involved. This need is quite common though from hot rodders and other vintage cars. Google “digital to mechanical speedometer” and you’ll find quite a few products. They’re not cheap but you’re paying for a solution.


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